1. Field of the Invention
The invention relates to infinitely variable traction roller transmissions in which motion is transmitted from a toric disc mounted on an input shaft to a toric disc mounted on an output shaft by traction rollers disposed between, and in engagement with, the toric discs.
To enable such toroidal traction roller transmissions to transmit large torques at high speeds for long life the traction rollers are positioned inwardly of the center of the toroidal cavity between the toric discs. Such an arrangement causes relatively little spin in the contact area of the traction rollers with the toric discs so that a relatively high traction coefficient and relatively little wear on the surfaces of the toric discs and the traction rollers are obtained. The contact forces necessary for engagement of the traction rollers with the toric discs are obtained by forcing the toric discs in engagement with the traction rollers.
2. Description of the Prior Art
Such traction roller transmissions are described in the applicant's U.S. Pat. No. 4,086,820 and also in applicant's earlier U.S. Pat. No. 3,810,398 wherein a rotary motion transmitting device is provided in which the support structures of the power rollers are interconnected by tension means which extend between the toric discs and balance the forces and vibrations exerted on the power rollers while allowing nearly friction free transverse motion of the roller assemblies for transverse shifting of the roller assembly for transmission ratio changing pivoting of the roller support structure. For engagement of the traction rollers with the toric discs, the toric discs are forced toward each other by cam structures generating load-dependent contact forces. Providing the proper amount of contact forces has been quite problematic especially as the cam structures are subject to wear after some time of use. If a drive is operated at a fairly constant load, its loading cam structure remains in an essentially constant position. Over an extended period, vibrations in the drive have a fretting action on the cam ramp surfaces which causes local changes in the ramp angle resulting in a change of the contact forces.
Also, the axial force provided by the cam structure is directly proportional to the torque applied but often this is not the proper match for the required contact forces of the drive because of the drive geometry. At the points of the greatest transmission ratios, the traction rollers are in contact with one of the toroidal discs on a surface area which has a relatively small angle with the axis of the discs so that wedging between the traction rollers and the toroidal discs takes place resulting in relatively high contact forces. Further the contact forces are not purely axial; they rather have a radial component which may be substantial depending on the angle between the axis and the contact surface area of the toroidal discs and which may result in a slight radial deflection of the traction roller support structure. This again will allow further increased axial movement of the traction discs. Axial movement of the traction discs in turn causes the traction rollers to move sidewise on their support structures in order to maintain equal axial forces at the two contact points of each roller. Since, however, at high transmission ratio the rollers are in contact with the respective disc at a small diameter area, this movement allows even further increased axial movement of the discs toward ech other. As a result, there is a considerable cam rise needed in order to prevent snapping over of the cam structures. Many drives have therefore two cam structures, one at the input and one at the output shaft. This however may only aggravate the problem since the torques of the input and output shafts are very different at the end points of the transmission ratio changes and, naturally, the cam structure exposed to the smaller torque will not respond while the cam structure exposed to the greater torque cannot provide for the axial movement of both cam structures.